Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison withInfrared Space ObservatoryLWS Observations

Abstract

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are Ðtted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with signiÐcant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by Ñared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely.

A reconsideration of disk properties in Herbig Ae stars

“…The thin line shows the AB Aur profile, that we repeat on each panel to facilitate the comparison optically thin, hotter surface layer, often referred to as the disk atmosphere (Calvet et al 1991), which does not affect the physical structure of the disk, but dominates the SED in the mid-infrared. The CG97 models need to specify a large number of parameters which are not well constrained by the SED alone (Chiang et al 2001). However, some of these parameters can be derived from independent observations, namely the stellar properties, the disk inclination θ (Table 1) and the disk outer radius, which we have taken equal to the observed millimeter continuum size.…”

“…About 1/3 of the cosmic abundance of C and all Si is locked into grains. We have followed Chiang et al (2001) in computing the heating of the disk midplane caused by a mixture of grains in the atmosphere. The adopted parameters are summarized in Table 4.…”

“…Chiang et al (2001) have recently presented an improved version of the CG97 disk models, where a more realistic dust model is adopted. They show that ices are important contributors to the outer disk opacity, and that taking into account intermolecular translational modes of water ice at 45, 62 and 154 µm improves the fit to the CQ Tau SED.…”

“…As a consequence, the spectral energy distributions (SED) of HAe stars have been interpreted as the emission of extended spherical envelopes of low optical depth (Pezzuto et al 1997;Miroshnichenko et al 1997;Il'in & Krivova 2000), of circumstellar disks (Hillenbrand et al 1995;Chiang et al 2001), or of a combination of both (Natta et al 1993;Miroshnichenko et al 1999). Given the number of free parameters and the fact that the SED is not uniquely defined, all the fits are equally good (or equally bad).…”

Circumstellar disks and planets

“…Is it optically thin or thick?) as a result of numerous ambiguities between the various physical parameters that describe a given disk (Chiang et al, 2001). To give a simple example, determining that a debris disk system is characterized by a black-body excess emission only firmly establishes the temperature of this dust population.…”